Voltage source converter

ABSTRACT

A Voltage Source Converter having at least one phase leg connected to opposite poles of a direct voltage side of the converter and comprising a series connection of switching elements including at least one energy storing capacitor and configured to obtain two switching states, namely a first switching state and a second switching state, in which the voltage across said at least one energy storing capacitor and a zero voltage, respectively, is applied across the terminals of the switching element, has semiconductor chips of said switching elements arranged in stacks comprising each at least two semiconductor chips. The converter comprises an arrangement configured to apply a pressure to opposite ends of each stack.

TECHNICAL FIELD OF THE INVENTION AND BACKGROUND ART

The present invention relates to a Voltage Source Converter having atleast one phase leg connecting to opposite poles of a direct voltageside of the converter and comprising a series connection of switchingelements, each said switching element having on one hand at least twosemiconductor chips having each a semiconductor device of turn-off typeand a free-wheeling diode connected in parallel therewith and on theother at least one energy storing capacitor, a mid point of said seriesconnection forming a phase output being configured to be connected to analternating voltage side of the converter and to divide the phase leginto an upper valve branch and a lower valve branch, each said switchingelement being configured to obtain two switching states by control ofsaid semiconductor devices of each switching element, namely a firstswitching state and a second switching state, in which the voltageacross said at least one energy storing capacitor and a zero voltage,respectively, is applied across the terminals of the switching element,for obtaining a determined alternating voltage on said phase output.

Such converters with any number of said phase legs are comprised, butthey have normally three such phase legs for having a three phasealternating voltage on the alternating voltage side thereof.

A Voltage Source Converter of this type may be used in all kinds ofsituations, in which direct voltage is to be converted into alternatingvoltage or conversely, in which examples of such uses are in stations ofHVDC-plants (High Voltage Direct Current), in which direct voltage isnormally converted into a three-phase alternating voltage or conversely,or in so-called back-to-back-stations in which alternating voltage isfirstly converted into direct voltage and this is then converted intoalternating voltage. It may also be used to absorb or inject reactivepower in the alternating voltage network.

A Voltage Source Converter of this type is known through for example DE101 03 031 A1 and WO 2007/023064 A1 and is normally called a multi-cellconverter or M2LC. Reference is made to these publications for thefunctioning of a converter of this type. Said switching elements of theconverter may have other appearances than those shown in saidpublications, and it is for instance possible that each switchingelement has more than one said energy storing capacitor, as long as itis possible to control the switching element to be switched between thetwo states mentioned in the introduction.

The present invention is primarily, but not exclusively, directed tosuch Voltage Source Converters configured to transmit high powers, andthe case of transmitting high powers will for this reason mainly bediscussed hereafter for illuminating but not in any way restricting theinvention thereto. When such a Voltage Source Converter is used totransmit high powers this also means that high voltages are handled, andthe voltage of the direct voltage side of the converter is determined bythe voltages across said energy storing capacitors of the switchingelements and is normally set to be half the sum of these voltages. Thismeans that a comparatively high number of such switching elements are tobe connected in series or a high number of semiconductor devices, i.e.said semiconductor chips, are to be connected in series in each saidswitching element, and a Voltage Source Converter of this type isparticularly interesting when the number of the switching elements insaid phase leg is comparatively high, such as at least 8. A high numberof such switching elements connected in series means that it will bepossible to control these switching elements to change between saidfirst and second switching state and already at said phase output obtainan alternating voltage being very close to a sinusoidal voltage. Thismay be obtained already by means of substantially lower switchingfrequencies than typically used in known Voltage Source Converters ofthe type shown in FIG. 1 in DE 101 03 031 A1 having switching elementswith at least one semiconductor device of turn-off type and at least onefree wheeling diode connected in anti-parallel therewith. This makes itpossible to obtain substantially low losses and also considerablyreduces problems of filtering and harmonic currents and radiointerference, so that equipment therefor may be less costly.

However, the great number of switching elements connected in series andthe energy storing capacitors belonging to these switching elementsmakes Voltage Source Converters of this type rather voluminous, so thatfor instance in the case of a station of a HVDC-plant very large valvehalls are to be built for such converters.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a Voltage SourceConverter of the type defined in the introduction being improved in atleast some aspect with respect to such Voltage Source Converters alreadyknown.

This object is according to the invention obtained by providing such aVoltage Source Converter, in which said semiconductor chips of saidswitching elements are arranged in stacks comprising each at least twosemiconductor chips, and that the converter comprises an arrangementconfigured to apply a pressure to opposite ends of each said stack forpressing said chips towards each other so as to obtain electric contactbetween semiconductor chips in said stack.

By using this so-called presspack technique known through U.S. Pat. No.5,705,853 this type of Voltage Source Converters may be made morecompact than before, so that the dimensions may be reduced forespecially buildings in the form of valve halls for such converters. Thesemiconductor chips in converters of this type have so far beenconnected by screw connections, which require more place for providingaccess to said screws or bolts for tightening thereof. Obtaining theelectric contact between semiconductor chips by arranging them in astack and pressing them towards each other also results in an increasedreliability of such connections with respect to prior solutions.

According an embodiment of the invention said arrangement comprisesmeans adapted to apply a spring loaded pressure to each said stackurging the two ends of the stack towards each other while releasingpotential energy stored in members of said means. Said members may be ofany type storing potential energy when compressed and are according toanother embodiment of the invention springs acting on at least one endof each said stack, in which said springs may be mechanical springs aswell as other types of springs, such as gas springs. This means that theelectric contact between the semiconductor chips in said stack may beobtained with a high reliability irrespectively of irregularities in thedimension thereof, such as for instance in the case of parallelconnection of semiconductor chips in said stack.

According to another embodiment of the invention said arrangementcomprises two end plates configured to be arranged close to oppositeends of a said stack and elongated members interconnecting said plateswhile determining the distance therebetween, and said members storingpotential energy are arranged to act between at least one of said platesand the corresponding end of said stack for urging this end plate andstack end apart while pressing the stack together. Said arrangement maythen comprise means configured to allow a displacement of at least oneof said plates along said interconnecting members in the longitudinaldirection thereof for changing said distance and by that the pressureapplied to said stack, so that the same equipment in the form of saidarrangement may be used and adapted to different such stacks.

According to another embodiment of the invention said arrangementcomprises a further plate configured to be applied onto one end of saidstack and movable with respect to said elongated members in thelongitudinal direction thereof, and said members storing potentialenergy are arranged to urge said further plate and a said end plate nextthereto apart for pressing the stack together.

According to another embodiment of the invention said semi-conductorchips have a plate-like structure and are arranged with large sidesthereof directed in the direction of the extension of the stack.

According to another embodiment of the invention said at least twosemiconductor chips belong to the same switching element, and adjacentsemiconductor chips belonging to the same switching element areseparated by a metal plate sandwiched therebetween for obtaining anelectrical connection between the two chips by pressing them againstsaid metal plate. This means that a reliable and excellent electriccontact may be obtained between said semiconductor chips belonging tothe same switching element.

According to another embodiment of the invention said metal plates arearranged on both sides of each said semiconductor chip, which isparticularly preferable for the possibility to use such metal plates forcooling said semiconductor chips.

According to another embodiment of the invention each said stackcomprises at least all semiconductor chips belonging to one switchingelement.

According to another embodiment of the invention each said stackcomprises said semiconductor chips of a plurality of said switchingelements, which makes the converter very compact.

According to another embodiment of the invention each said stack has thesemiconductor chips of one switching element arranged in a sub-stack,all said sub-stacks to be pressed together by one and the same saidarrangement are arranged on top of each other in one single stack, anelectrically insulating layer is sandwiched between and separatesadjacent such sub-stacks, and a conductor is arranged for electricallyconnecting adjacent sub-stacks and by that adjacent switching elementsin said series connection to each other. This design of said stack makesit possible to arrange a number of switching elements, even allswitching elements of a said valve branch, in one single stack makingthe converter very compact.

According to another embodiment of the invention said semi-conductorchips to be pressed together by one and the same said arrangement arearranged in at least two parallel stacks, each said parallel stackcomprises a plurality of superimposed switching elements each havingsaid semiconductor chips thereof arranged in a sub-stack, all saidsub-stacks of each of said stacks are arranged on top of each other forforming one of said parallel stacks, an electrically insulating layer issandwiched between and separates adjacent such sub-stacks, eachsub-stack switching element comprises two metal plates separated by atleast one semiconductor chip of this switching element, and saidparallel stacks are mutually displaced in the longitudinal directionthereof, so that for each switching element said two metal platesbelonging thereto connects to and are in common to different adjacentswitching elements of the other parallel stack so as to obtain a seriesconnection of two switching elements separated by a said insulatinglayer in one said parallel stack with a switching element of the otherparallel stack arranged in this series connection between said twoswitching elements. This way of obtaining a series connection of saidswitching elements or cells in a zigzag-like pattern makes it possibleto make the converter even more compact and reduce the size (length) ofvalve buildings.

According to another embodiment of the invention said metal plates areprovided with channels and the converter comprises means configured tocirculate a cooling medium in said channels for cooling saidsemiconductor chips adjacent to said metal plates, in which said coolingmedium is preferably water, although other types of cooling media areconceivable. The use of this cooling technique results in a furtheradvantage of the embodiment defined above having parallel stacks, sincethis means that the number of connections of cooling medium to theswitching elements of the converter may be reduced to the half of thenumber required would the switching elements thereof not share suchmetal plates.

According to another embodiment of the invention each switching elementcomprises more than two said semiconductor chips arranged in a saidstack. An advantage of having a larger number of semiconductor chips ineach switching element is that costs may be saved due to a lower numberof connections required to switching elements as a consequence of alower number of switching elements. However, it is a trade off betweenthis advantage and the advantage of a better quality of the alternatingvoltage obtained on said phase output would the number of switchingelements or cells be higher.

According to another embodiment of the invention each said switchingelement has 2N said semiconductor chips following upon each other in asaid stack, in which N is an integer 2.

According to another embodiment of the invention the number of theswitching elements of said phase leg is ≧8, 12-32, 16-24 or 50-150. Aconverter of this type is, as already mentioned above, particularlyinteresting when the number of switching elements of a said phase leg israther high resulting in a high number of possible levels of the voltagepulses delivered on said phase output.

According to another embodiment of the invention said semi-conductordevices of the switching element chips are IGBTs (Insulated Gate BipolarTransistor) or GTOs (Gate Turn-Off thyristor). These are suitablesemiconductor devices for such converters, although other semiconductordevices of turn-off type, such as IGCTs are also conceivable.

According to another embodiment of the invention said converter isconfigured to have said direct voltage side connected to a directvoltage network for transmitting High Voltage Direct Current (HVDC) andthe alternating voltage side connected to an alternating voltage phaseline belonging to an alternating voltage network. This is due to thehigh number of semiconductor chips required a particularly interestingapplication of a converter of this type.

According to another embodiment of the invention the converter isconfigured to have a direct voltage across said two poles being 1kV-1200 kV, 10 kV-1200 kV or 100 kV-1200 kV. The invention is the moreinteresting the higher said direct voltage is.

The invention also relates to a plant for transmitting electric poweraccording to the appended claim therefor. The size of the stations ofsuch a plant may be reduced with respect to such plants already knownusing a Voltage Source Converter of the type defined in theintroduction.

Further advantages as well as advantageous features of the inventionwill appear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a description ofembodiments of the invention cited as examples.

In the drawings:

FIG. 1 is a very simplified view of a Voltage Source Converter of thetype according to the present invention,

FIG. 2 and FIG. 3 illustrates two different known switching elements,which may be a part of a Voltage Source Converter according to theinvention,

FIG. 4 is a simplified view very schematically illustrating a VoltageSource Converter according to the present invention,

FIG. 5 is a simplified view illustrating how two switching elements maybe superimposed in one single stack in a converter according to a firstembodiment of the present invention,

FIG. 6 is a simplified view illustrating the principle of obtaining andarranging stacks of switching elements of the type shown in FIG. 5 in aconverter according to said first embodiment of the invention,

FIG. 7 is a very simplified view corresponding to FIG. 5 of a part of aso-called parallel stack of switching elements of a converter accordingto a second embodiment of the invention,

FIG. 8 is a circuit diagram of the part of said parallel stack shown inFIG. 7,

FIG. 9 is a circuit diagram illustrating a switching element of aconverter according to a third embodiment of the invention,

FIG. 10 is a very simplified view illustrating the switching elementaccording to FIG. 9, and

FIG. 11 is a very simplified view from above of the switching elementshown in FIG. 10.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates very schematically the general construction of aVoltage Source Converter 1 of the type to which the present inventionrelates. This converter has three phase legs 2-4 connected to oppositepoles 5, 6 of a direct voltage side of the converter, such as a directvoltage network for transmitting high voltage direct current. Each phaseleg comprises a series connection of switching elements 7 indicated byboxes, in the present case 16 to the number, and this series connectionis divided into two equal parts, an upper valve branch 8 and a lowervalve branch 9, separated by a mid point 10-12 forming a phase outputbeing configured to be connected to an alternating voltage side of theconverter. The phase outputs 10-12 may possibly through a transformerconnect to a three phase alternating voltage network, load, etc.Filtering equipment is also arranged on said alternating voltage sidefor improving the shape of the alternating voltage on said alternatingvoltage side.

A control arrangement 13 is arranged for controlling the switchingelements 7 and by that the converter to convert direct voltage intoalternating voltage and conversely.

The Voltage Source Converter has switching elements 7 of the type havingon one hand at least two semiconductor chips with each a semiconductordevice of turn-off type, and a free wheeling diode connected in paralleltherewith and on the other at least one energy storing capacitor, andtwo examples of such switching elements are shown in FIG. 2 and FIG. 3.The terminals 14, 15 of the switching element are adapted to beconnected to adjacent switching elements in the series connection ofswitching elements forming a phase leg. The semiconductor devices 16, 17are in this case IGBTs connected in parallel with diodes 18, 19. Anenergy storing capacitor 20 is connected in parallel with the respectiveseries connection of the diodes and the semiconductor devices. Oneterminal 14 is connected to the mid point between the two semiconductordevices as well as the mid point between the two diodes. The otherterminal 15 is connected to the energy storing capacitor 20, in theembodiment of FIG. 2 to one side thereof and in the embodiment accordingto FIG. 3 to the other side thereof. It is pointed out that eachsemi-conductor device and each diode as shown in FIG. 2 and FIG. 3 maybe more than one connected in series for being able to handle thevoltages to be handled, and the semiconductor devices so connected inseries may then be controlled simultaneously so as to act as one singlesemiconductor device.

The switching elements shown in FIG. 2 and FIG. 3 may be controlled toobtain one of a) a first switching state and b) a second switchingstate, in which for a) the voltage across the capacitor 20 and for b) azero voltage is applied across the terminals 14, 15. For obtaining thefirst state in FIG. 2 the semiconductor device 16 is turned on and thesemiconductor device 17 turned off and in the embodiment according toFIG. 3 the semiconductor device 17 is turned on and the semiconductor 16is turned off. The switching elements are switched to the second stateby changing the state of the semiconductor devices, so that in theembodiment according to FIG. 2 the semiconductor device 16 is turned offand 17 turned on and in FIG. 3 the semiconductor device 17 is turned offand 16 turned on.

FIG. 4 shows a little more in detail how a phase leg of the converteraccording to FIG. 1 is formed by switching elements of the type shown inFIG. 3, in which totally ten switching elements have been left out forsimplifying the drawing. The control arrangement 13 is adapted tocontrol the switching elements by controlling the semiconductor devicesthereof, so that they will either deliver a zero voltage or the voltageacross the capacitor to be added to the voltages of the other switchingelements in said series connection. A transformer 21 and filteringequipment 22 are here also indicated. It is shown how each valve branchis through a phase reactor 50, 51 connected to the phase output 10, andsuch phase reactors should also be there in FIG. 1 for the phase outputs10, 11 and 12, but have there been left out for simplifying theillustration.

FIG. 5 illustrates very schematically a part of a stack in the form oftwo switching elements 7′ superimposed of the type shown in FIG. 2. Eachswitching element 7′ comprises two semiconductor chips 30, 31 havingeach a semiconductor device of turn-off type and a free-wheeling diodeconnected in parallel therewith and having a plate-like structure with ametal plate 32-34 on each side of each semiconductor chip in a sub-stack35 so formed. It is illustrated how switching elements following uponeach other in said stack are electrically insulated with respect to eachother through an insulating layer 36 separating the metal plates 34 and32. Adjacent switching elements are connected to each other by anelectrical conductor 37 in the form of a wire.

It is illustrated how ducts 38 transporting a cooling medium, such ascooling water, are connected to channels in the metal plates 32-34 forcooling the semiconductor chips located between these metal plates. Inthe present case the water is led through the plates 32, 33 and 34 forobtaining a cooling effect thereupon with the relationship 9:10:1, whichindicates the cooling need of the different metal plates. Such coolingis of course provided for all the metal plates in the switching elementsin the converter, although it has only been shown for the lowerswitching element in FIG. 5 for simplifying the drawings. This is ofcourse also valid for example the energy storing capacitors 20 and theelectrical conductor 37, which are only shown for one switching element.

FIG. 6 illustrates how four such sub-stacks 35 of switching elements 7′according to FIG. 5 may be arranged in one single stack and providedwith an arrangement 39 configured to apply pressure to opposite ends 40,41 of the stack S for pressing the semiconductor chips 30, 31 towardsthe respective metal plates and towards each other so as to obtainelectric contact between the semiconductor chips in the same sub-stack.This arrangement 39 comprises means 42 adapted to apply a spring loadedpressure to each said stack. The arrangement has two end plates 43, 44configured to be arranged close to opposite ends of the stack andelongated members 45 in the form of rods, for instance of glass fibres,interconnecting the plates 43, 44 while determining the distancetherebetween. The plates 43, 44 may be displaced with respect to eachother by tightening or releasing nuts 46 located on threaded ends of therods 45. A further plate 47 is configured to be applied onto one end ofthe stack and is movable with respect to the rods 45 in the longitudinaldirection thereof. Spring members 48 storing potential energy arearranged to urge said further plate 47 and the end plate 44 next theretoapart for pressing the stack together. This results in a very reliablemutual contact of the semiconductor chips in the stack.

Only the upper valve branch 8 of the converter and the phase output 10is shown in FIG. 6, and this converter has accordingly 8 switchingelements connected in series in each valve branch. Another number ofswitching elements are of course conceivable, and these may be dividedinto a number of stacks being judged to be most appropriate for therespective application. It would for instance be possible to have allthe switching elements of the valve branch arranged in one single stackheld together by one single said arrangement 39. This way of arrangingthe switching elements makes the arrangement of the semiconductor chipsthereof very compact with the possibility to keep the dimensions ofvalve halls down.

It is shown through the circle 50 to the right in FIG. 6 how forinstance four semiconductor chips 30 may be arranged in parallel betweeneach metal plate 32-34 of a switching element for being able to togethertake the current that may flow therethrough. Thus, the semiconductorchips 30, 31 shown in FIG. 5 and also FIGS. 6 and 7 may stand for such aparallel connection of a plurality of semiconductor chips.

FIG. 7 illustrates schematically how switching elements in a converteraccording to a second embodiment of the invention may be arranged in aso-called “double” stack, and the circuit diagram thereof is shown inFIG. 8. Each of two parallel stacks 51, 52 comprises a plurality ofsuperimposed switching elements 7 b, 7 d and 7 a, 7 c, respectively,each having semiconductor chips 30, 31 thereof arranged in a sub-stack.An electrically insulating layer 53 is sandwiched between and separatesadjacent such sub-stacks, here arranged on the collector side of thesemiconductor chip 30. Each sub-stack switching element comprises twometal plates 54, 55 extending to the other sub-stack. The parallelstacks 51, 52 are mutually displaced in the longitudinal directionthereof, so that for each switching element said two metal plates 54, 55belonging thereto connect to and are in common to different adjacentswitching elements of the other parallel stack so as to obtain a seriesconnection of two switching elements separated by a said insulatinglayer 53 in one said parallel stack with a switching element of theother parallel stack arranged in the series connection between said twoswitching elements. This means that the two metal plates 54, 55 of theswitching element 7 b also belong to the switching elements 7 a and 7 c,respectively. This results in a series connection of the switchingelements in the parallel stacks according to a zigzag-like pattern inthe order 7 a, 7 b, 7 c and 7 d.

The parallel stacks 51, 52 shown in FIG. 7 may contain any appropriatenumber of superimposed switching elements and are held together by onearrangement of the type shown in FIG. 6 pressing the semiconductor chipsthereof towards the metal plates. An advantage of this design withrespect to the design shown in FIG. 5 is that the height of the stackmay for a determined number of switching elements connected in series bereduced and the number of cooling medium (water) connections to metalplates will also be reduced.

FIG. 9 illustrates schematically a switching element in a converteraccording to a third embodiment of the invention. This switching elementhas totally 16 semiconductor chips 30, 31 connected in series. Gatedrive units 60 used to control the respective semi-conductor device ofthe semiconductor chips are schematically illustrated. An externalcapacitor voltage divider 61 utilizing two voltage divider resistors 62,63 for measuring the total voltage across the capacitors 20+20′ as wellas the difference of the voltages U₂₀ and U_(20′) across thesecapacitors is arranged for detecting possible faults in any of saidcapacitors. The capacitor voltage measurement is also used in thecontrol of the voltage division between switching elements in the samebranch.

The switching element formed by 16 semiconductor chips arranged in onestack with metal plates sandwiched therebetween and held together by anarrangement 39 of the type described above is very schematicallyillustrated in FIG. 10. FIG. 11 shows the switching element from above,in which it is schematically indicated by arrows 70 how the capacitorsare connected to the semiconductor chips. Such a switching element maytypically have a voltage (U₂₀+U_(20′)) in the order of 20 kV across thecapacitors thereof.

Such a larger number of semiconductor chips in the same switchingelement or cell results in a reduced number of switching elements in theconverter, so that costs with respect to connections to the convertermay be saved. However, this also means a lower number of differentlevels possible to obtain for pulses on said phase output, so that thealternating voltage resulting from said conversion will have a lowerquality.

The invention is of course not in any way restricted to the embodimentsdescribed above, but many possibilities to modifications thereof will beapparent to a person with ordinary skill in the art without departingfrom the basic idea of the invention as defined in the appended claims.

“Plates” as used in this disclosure for the members of the arrangementpressed against opposite ends of the stacks is to be interpreted broadlyand also covers more box-like members and members having different typesof recesses, hollow spaces and the like.

1-21. (canceled)
 22. A multi-cell Voltage Source Converter having atleast one phase leg connecting to opposite poles of a direct voltageside of the converter and comprising a series connection of switchingelements, each said switching element having on one hand at least twosemiconductor chips and on the other at least one energy storingcapacitor, a mid point of said series connection forming a phase outputbeing configured to be connected to an alternating voltage side of theconverter and to divide the phase leg into an upper valve branch and alower valve branch, wherein said semi-conductor chips of said switchingelements are arranged in stacks comprising each at least twosemi-conductor chips, and the converter comprises an arrangementconfigured to apply a pressure to opposite ends of each said stack forpressing said chips towards each other so as to obtain electric contactbetween semiconductor chips in said stack, wherein said semiconductorchips have a plate-like structure and are arranged with the large sidesthereof directed in the direction of the extension of the stack, atleast two semiconductor chips belong to the same switching element andadjacent semiconductor chips belonging to the same switching element areseparated by a metal plate sandwiched therebetween for obtaining anelectrical connection between the two chips by pressing them againstsaid metal plate.
 23. A converter according to claim 22, wherein saidarrangement comprises means adapted to apply a spring loaded pressure toeach said stack urging the two ends of the stack towards each otherwhile releasing potential energy stored in members of said means.
 24. Aconverter according to claim 23, wherein said members are springs actingon at least one end of each said stack.
 25. converter according to claim23, wherein said arrangement comprises two end plates configured to bearranged close to opposite ends of a said stack and elongated membersinterconnecting said plates while determining the distance therebetween,and said members storing potential energy are arranged to act between atleast one of said plates and the corresponding end of said stack forurging this end plate and stack end apart while pressing the stacktogether.
 26. A converter according to claim 25, wherein saidarrangement comprises means configured to allow a displacement of atleast one of said plates along said interconnecting members in thelongitudinal direction thereof for changing said distance and by thatthe pressure applied to said stack.
 27. A converter according to claim25, wherein said arrangement comprises a further plate configured to beapplied onto one end of said stack and movable with respect to saidelongated members in the longitudinal direction thereof, and saidmembers storing potential energy are arranged to urge said further plateand a said end plate next thereto apart for pressing the stack together.28. A converter according to claim 22, wherein said metal plates arearranged on both sides of each said semiconductor chip.
 29. A converteraccording to claim 22, wherein each said stack comprises at least allsemiconductor chips belonging to one switching element.
 30. A converteraccording to claim 22, wherein each said stack comprises saidsemiconductor chips of a plurality of said switching elements.
 31. Aconverter according to claim 22, wherein each said stack has thesemiconductor chips of one switching element arranged in a sub-stack,all said sub-stacks to be pressed together by one and the same saidarrangement are arranged on top of each other in one single stack, anelectrically insulating layer is sandwiched between and separatesadjacent such sub-stacks, and a conductor is arranged for electricallyconnecting adjacent sub-stacks and by that adjacent switching elementsin said series connection to each other.
 32. A converter according toclaim 30, wherein said semiconductor chips to be pressed together by oneand the same said arrangement are arranged in at least two parallelstacks, each said parallel stack comprises a plurality of superimposedswitching elements each having said semiconductor chips thereof arrangedin a sub-stack, all said sub-stacks of each of said parallel stacks arearranged on top of each other for forming one of said parallel stacks,an electrically insulating layer is sandwiched between and separatesadjacent such sub-stacks, each sub-stack switching element comprises twometal plates separated by at least one semiconductor chip of thisswitching element, and said parallel stacks are mutually displaced inthe longitudinal direction thereof, so that for each switching elementsaid two metal plates belonging thereto connects to and are in common todifferent adjacent switching elements of the other parallel stack so asto obtain a series connection of two switching elements separated by asaid insulating layer in one said parallel stack with a switchingelement of the other parallel stack arranged in this series connectionbetween said two switching elements.
 33. A converter according to claim28, wherein said metal plates are provided with channels and theconverter comprises means configured to circulate a cooling medium insaid channels for cooling said semiconductor chips adjacent to saidmetal plates.
 34. A converter according to claim 22, wherein eachswitching element comprises more than two said semiconductor chipsarranged in a said stack.
 35. A converter according to claim 34, whereineach said switching element has 2N said semiconductor chips followingupon each other in a said stack, in which N is an integer>2.
 36. Aconverter according to claim 22, wherein the number of the switchingelements of said phase leg is >8, 12-32, 16-24 or 50-150.
 37. Aconverter according to claim 22, wherein said semiconductor devices ofthe switching element chips are IGBTs (Insulated Gate BipolarTransistor) or GTOs (Gate Turn-Off Thyristor).
 38. A converter accordingto claim 22, wherein it is configured to have said direct voltage sideconnected to a direct voltage network for transmitting High VoltageDirect Current (HVDC) and the alternating voltage side connected to analternating voltage phase line belonging to an alternating voltagenetwork.
 39. A converter according to claim 22, wherein it is configuredto have a direct voltage across said two poles being 1 kV-1200 kV, 10kV-1200 kV or 100 kV-1200 kV.
 40. A plant for transmitting electricpower comprising a direct voltage network and at least one alternatingvoltage network connected thereto through a station, said station beingadapted to perform transmitting of electric power between the directvoltage network and the alternating voltage network and comprises atleast one Voltage Source Converter adapted to convert direct voltageinto alternating voltage and conversely, wherein said station of theplant comprises a Voltage Source Converter according to claim
 22. 41.converter according to claim 24, wherein said arrangement comprises twoend plates configured to be arranged close to opposite ends of a saidstack and elongated members interconnecting said plates whiledetermining the distance therebetween, and said members storingpotential energy are arranged to act between at least one of said platesand the corresponding end of said stack for urging this end plate andstack end apart while pressing the stack together.